I. Field of the Invention
The present invention relates to vocoders. More particularly, the present invention relates to a novel and improved method for determining speech encoding rate in a variable rate vocoder.
II. Description of the Related Art
Variable rate speech compression systems typically use some form of rate determination algorithm before encoding begins. The rate determination algorithm assigns a higher bit rate encoding scheme to segments of the audio signal in which speech is present and a lower rate encoding scheme for silent segments. In this way a lower average bit rate will be achieved while the voice quality of the reconstructed speech will remain high. Thus to operate efficiently a variable rate speech coder requires a robust rate determination algorithm that can distinguish speech from silence in a variety of background noise environments.
One such variable rate speech compression system or variable rate vocoder is disclosed in copending U.S. patent application Ser. No. 07/713,661 filed Jun. 11, 1991, entitled "Variable Rate Vocoder" and assigned to the assignee of the present invention, the disclosure of which is incorporated by reference. In this particular implementation of a variable rate vocoder, input speech is encoded using Code Excited Linear Predictive Coding (CELP) techniques at one of several rates as determined by the level of speech activity. The level of speech activity is determined from the energy in the input audio samples which may contain background noise in addition to voiced speech. In order for the vocoder to provide high quality voice encoding varying levels of background noise which may affect the speech activity level detection and rate determination, an adaptively adjusting threshold technique is used to compensate for the affect of background noise on rate decision.
Vocoders are typically used in communication devices such as cellular telephones or personal communication devices to provide digital signal compression of an analog audio signal that is converted to digital form for transmission. In a mobile environment in which a cellular telephone or personal communication device may be used, high levels of background noise energy make it difficult for the rate determination algorithm to distinguish low energy unvoiced sounds from background noise silence using a signal energy based rate determination algorithm. Thus unvoiced sounds frequently get encoded at lower bit rates and the voice quality becomes degraded as consonants such as "s", "x", "ch", "sh", "t", etc. are lost in the reconstructed speech.
It is therefore an object of the present invention to provide in a variable rate vocoder an improvement in rate determination for unvoiced speech.
It is yet another object of the present invention to provide a technique for distinguishing low energy unvoiced speech from background noise in a variable rate vocoder in which rate determination is based upon signal energy to provide improved quality in the vocoded speech.